1. Field of the Invention
The present invention relates to a moisture-curable hot-melt adhesive and more particularly to a moisture-curable hot-melt adhesive composition which requires only a comparatively low temperature for the adhesion of automotive window glass and other adherends and gives a bond which is highly resistant to heat and has improved initial physical properties in terms of strength, elongation and modulus.
2. Brief Description of the Prior Art
As a known moisture-curable hot-melt adhesive, there is a prepolymer composition obtainable by reacting a thermoplastic saturated polyester polyol having a molecular weight not exceeding 6,000 with an isocyanate compound. This composition is low in melting temperature and can be cured at a low temperature. Moreover, while the cured composition has satisfactory heat resistance, it is unsatisfactory in initial physical properties.
By way of illustration, the conventional moisture-curable one-package urethane adhesive used in the automotive production line will not be completely cured even when the finished automobile rolls out at the end of the assembly line, and is frequently subjected to various impacts and vibrations, for example due to opening and closing of the doors, in the uncured condition. Furthermore, when window glass, for instance, is set up in position on the production line and the finished automobile is stocked outdoors within a brief time of, say, 2 hours after bonding, or when the automobile is stocked outdoors in the dry winter months, the adhesive is hardly allowed to cure. Particularly for production lines located in frigid areas, a moisture-curable hot-melt adhesive providing a satisfactory bond with sufficient initial physical properties at comparatively low temperatures is keenly required.
The object of the present invention is to provide a moisture-curable hot-melt adhesive composition which insures a sufficiently tough bond at low temperatures and excellent initial physical characteristics, and a method for production of such adhesive composition.
The inventors of the present invention found that when a thermoplastic urethane prepolymer is synthesized by reacting an excess of a polyisocyanate compound with a polyol component consisting of a high molecular polyester polyol containing a soft segment and a low molecular polyol compatible with said soft segment, the high molecular weight polyol contributes to the early onset of initial physical properties, while the low molecular polyol acts to lower the melting temperature of the composition, with the additional effect of improved adhesion, thus allowing one to take advantage of the inherent physical properties of thermoplastic polyester resins.